With the direct injection of the fuel into a combustion chamber of an internal combustion engine, it is possible in particular to realize a stratified combustion chamber charge. This can contribute significantly to the dethrottling of the Otto-cycle working process because the internal combustion engine can be leaned to a very great extent by means of the stratified charge operation, which offers thermodynamic advantages in particular in part-load operation (in the lower and middle load range, when small amounts of fuel are to be injected).
However, in the case of the direct injection of fuel into internal combustion engines, problems may be caused by the coking of the injection device, for example of an injection nozzle which is used for the injection. Here, extremely small quantities of fuel which adhere to the injection device during the injection undergo incomplete combustion under oxygen-deficient conditions. Deposits of coking residues form on the injection device. Said coking residues may firstly disadvantageously change the geometry of the injection device and influence or hinder the formation of the injection jet, and thereby sensitively disrupt mixture preparation. Secondly, injected fuel accumulates in the porous coking residues, which fuel, often toward the end of the combustion when the oxygen provided for the combustion has been almost completely consumed, then undergoes incomplete combustion and forms soot, which in turn contributes to the increase in particle emissions. Furthermore, coking residues may become detached for example as a result of mechanical loading caused by a pressure wave propagating in the combustion chamber or the action of the injection jet. The residues detached in this way may lead to damage in the exhaust-gas discharge system, and for example impair the functional capability of exhaust-gas aftertreatment systems provided in the exhaust-gas discharge system.
Attempts to address the build-up of coking residues and/or deplete deposits of coking residues (e.g., remove said coking residues) include measures described by German laid-open specification DE 199 45 813 A1, European patent EP 1 404 955 B1, and German laid-open specification DE 101 17 519 A1, for example.
The German laid-open specification DE 199 45 813 A1 describes a method for operating a direct-injection internal combustion engine, in which method, upon the detection of deposits in the combustion chamber, for example on an injection valve, measures are implemented in a targeted manner for cleaning the combustion chamber, wherein the presence of deposits in the combustion chamber is inferred from a misfire detection system. Measures proposed for cleaning the combustion chamber include the targeted initiation of knocking combustion and/or the introduction of a cleaning fluid into the intake combustion air. Both measures must be regarded as critical with regard to fuel consumption and pollutant emissions. Proposed as a particularly advantageous cleaning fluid is water, the injection of which causes the combustion temperature to be lowered, as a result of which the emissions of nitrogen oxides (NOx) can be simultaneously reduced. The injection of water is however not suitable in part-load operation at low loads and low rotational speeds, because this harbors the risk of corrosion in the combustion chamber and in the exhaust-gas discharge system, and can yield disadvantages in terms of wear.
The European patent EP 1 404 955 B1 describes an internal combustion engine whose at least one combustion chamber has, at least in regions, a catalytic coating on the surface for the purpose of oxidation of coking residues. The catalytic layer is intended to promote the oxidation of coking residues, specifically to effect a fast oxidation of the carbon-containing lining at a boundary surface between the catalytic converter and lining at typical operating temperatures, and to thereby effect an early detachment of the deposit under the action of the prevailing flow. In this way, it is sought to reduce or even completely prevent growth of the residues. A disadvantage of the method described in EP 1 404 955 B1 for the reduction of coking residues by means of oxidation is that, even when using catalytic materials, the minimum temperatures required for the oxidation are not always reached in part-load operation at low loads and low rotational speeds. It is however precisely these operating conditions of the internal combustion engine, specifically low loads and/or low rotational speeds, that promote, that is to say expedite, the formation of deposits of the type in question, and that necessitate a method for removing said deposits.
The German laid-open specification DE 101 17 519 A1 describes a method for operating a direct-injection internal combustion engine in which the inlet valve unit of a cylinder is purposely equipped with a means which prevents the dissipation of heat, that is to say is designed to increase the surface temperature in the region of the throat of the inlet valve. It is thereby sought to ensure that, at least in the throat, the high temperatures required for the depletion of coking residues are attained more often, or regularly, during normal operation of the internal combustion engine. Nevertheless, that region in the load-engine speed characteristic map in which the required temperatures are actually reached is merely widened, that is to say enlarged. The region in which the minimum temperatures of 380° C. required for the depletion of coking residues prevail lies close or adjacent to the full-load line at high engine speeds and high loads. Method-based measures for targetedly increasing the component temperature in other characteristic map regions are not implemented in DE 101 17 519 A1. Rather, it is relied upon that the required temperatures are generated of their own accord during normal operation of the internal combustion engine in corresponding regions of the load-engine speed characteristic map. In this respect, the method of DE 101 17 519 A1 also does not permit the depletion of coking residues, that is to say cleaning by means of oxidation, at low loads and low engine speeds of the internal combustion engine.
The above-described problem takes on an even greater significance during the warm-up phase of the internal combustion engine, in particular directly after a cold start of the internal combustion engine, when the component temperatures are particularly low. This is because the low temperature level not only expedites the formation of coking residues but also makes the removal of said residues more difficult.
In one example, the issues described above may be addressed by a method for operating an applied-ignition, direct-injection internal combustion engine having at least one cylinder, in which each cylinder is, for the direct introduction of fuel into the cylinder, equipped with an injection device which has a movable closure body, the method comprising: by positive control, during the course of an injection, connecting at least two openings provided in a housing of the injection device to a fuel supply system for the purposes of introducing fuel into the cylinder, and thus opening up the at least two openings by moving the movable closure body from a rest position in which the at least two openings are separated and blocked from the fuel supply system to a working position connecting the at least two openings to the fuel supply system; and proceeding from the working position in which the at least two openings are connected to the fuel supply system, moving the closure body from the working position into the rest position, with the at least two openings of the injection device being separated from the fuel supply system in succession with a time offset in such a way that at least one opening of the at least two openings is already fully separated from the fuel supply system while at least one other opening of the at least two openings is still connected to the fuel supply system. In this way, a likelihood of fuel deposits forming on a tip of the injection device may be reduced, and particle emissions from the engine may be decreased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.